Hydrogen-induced-cracking resistant and sulphide-stress-cracking resistant steel alloy

ABSTRACT

A steel alloy, for use in casing for oil and gas wells in low pH environments. The steel alloy has a carbon range by weight of 0.15% to 0.35%, a manganese range by weight of 0.60% to 1.10%, a molybdenum range by weight of 0.15% to 0.65%, a calcium range by weight of 0.0045% and a sulfur range by weight of less than 0.002%. The steel alloy also has, by weight, a chromium range of less than 0.50%, and an aluminum range of 0.010% to 0.080%.

RELATED APPLICATIONS

[0001] The present application is a continuation application of U.S. application Ser. No. 09/036,545, which is a continuation application of application Ser. No. 09/036,545 filed on Mar. 6, 1998. The '545 application is a continuation-in-part application of U.S. application Ser. No. 08/813,374 filed on Mar. 7, 1997. The present application incorporates by reference the entire contents of all of these parent applications.

FIELD OF THE INVENTION

[0002] This invention relates in general to a steel alloy and more particularly to a steel alloy that resists hydrogen-induced cracking and sulfide stress cracking. This alloy is particularly suitable for use in casing for use with sour oil and gas wells. The invention also comprises casing made from such alloy.

BACKGROUND TO THE INVENTION

[0003] In a number of different industries including the oil and gas industry, there is a need for steel alloys that can resist severe mechanical stresses, hydrogen-induced cracking (HIC), and sulfide stress cracking (SSC).

[0004] Oil and gas well casing is commonly exposed to highly acidic conditions. Many new wells contain significant fractions of H₂S while older wells become increasingly sour over their production lifetime. It is well known in the oil and gas industry that the presence of H₂S will act to promote HIC in well casing as HIC is exacerbated when well casing is used in low pH environments. In addition to resisting HIC, casing in mature oil wells may also be required to bear severe mechanical stresses; thus, susceptibility to SSC must also be considered.

[0005] As gas and oil reserves are depleted, improved methods of extracting gas and oil from mature reservoirs have developed apace. In particular, horizontal well technology has developed rapidly over the past decade as a means of enhancing recovery from mature gas and oil reserves. Well casing used in horizontal well technology for mature oil and gas wells is subjected to severe mechanical stresses, SSC, and HIC.

[0006] During the initial well stimulation portion of the horizontal well cycle, steam is pumped into the well, thereby heating the casing wall to temperatures as high as 350° C. However, the casing is unable to expand due to the physical constraints placed on the casing by the environment. Accordingly, a significant compressive stress is developed during this period of the cycle. This compressive stress will gradually relax through stress relaxation over an extended period of time. At the conclusion of the stimulation cycle, the well cools and the resultant thermal contraction of the material results in a tensile load. Again, this load may gradually relax over an extended period of time, so that when the next stimulation cycle is initiated, the casing again goes into compression. This cycling will be repeated several times through the life of the well and places severe fatigue stresses on the casing.

[0007] As was well known in the art prior to the time of the invention, HIC- and SSC-resistant casing alloys are generally made from seamless casing. In order to produce a pipe the alloy is cast as a billet and rolled into a solid round piece that is then pierced. Unfortunately, seamless casing is very expensive to manufacture. Further to this, the wall thickness may be irregular and segregation may occur on the centreline. which becomes the inner wall upon piercing. On the other hand, rolling and welding produces a casing that has a more uniform wall thickness. It is also less expensive to produce. Consequently, it would be preferable to manufacture the casing by rolling and welding the alloy.

[0008] Casing for horizontal oil wells should be made using alloys that can resist severe mechanical stresses, HIC and SSC. The IK55 alloy of Ipsco Inc. described hereinbelow, is one such alloy that had been used to make horizontal well casing.

[0009] The IK55 alloy is a medium-carbon quench-and-temper steel to which boron has been added to ensure hardening of the full thickness of the steel during the quenching treatment. To be effective, boron must be retained in solid solution throughout the processing schedule; however, boron interacts strongly with nitrogen to form boron nitrides which render boron additions ineffective. To prevent boron nitride formation, titanium is added to react with nitrogen in advance of any reaction between boron and nitrogen, thereby ensuring that boron is retained in solid solution.

[0010] Although steel alloys such as the IK55 alloy perform well in laboratory tests in which the pH is greater than about 4.25, if such alloys are exposed to low pH environments, environments having a pH lower than about 4.25, over a prolonged period of time, such alloys are subject to HIC. Accordingly, there is a need for a HIC resistant steel alloy suitable for environments having a pH of less than 4.25. However, even alloys that demonstrate good HIC resistance in laboratory trials may perform poorly when stress is applied. Thus, there is a need for alloys that demonstrate both good HIC resistance and good SSC resistance.

SUMMARY OF THE INVENTION

[0011] An object of one aspect of the present invention is to provide a steel alloy that provides resistance to HIC and SSC in low pH environments. The steel alloy is preferably rolled following casting.

[0012] In accordance with one aspect of the invention there is provided a steel alloy characterized in that the alloy has, by weight, a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, and a sulfur range of less than 0.002%. Molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy. This substantially precludes the formation of boron nitride and titanium nitride.

[0013] In various preferred embodiments of the invention, the range of each element of the alloy is more narrowly defined. The selection of the particular alloy chemistry, within the above specified limits, depends on trade-offs between a number of different factors such as the cost of the various alloying elements, as well as the HIC and SSC resistance required. In a first preferred alloy chemistry, the alloy has a carbon range by weight of 0.18 to 0.3%, a manganese range by weight of 0.6% to 1.10%, a molybdenum range by weight of 0.25 to 0.055%, and a sulfur range by weight of less than 0.001%. The above-defined preferred alloy chemistry is further definable to have a calcium range, by weight, of 0.0020% to 0.0045%, and an aluminum range of 0.010% to 0.080%. The alloy may also comprise silicon in a range of less than or equal to 0.30% by weight. Chromium in a range of 0.2 to 0.3% may further be added to the alloy.

[0014] In another preferred alloy chemistry, the carbon range by weight is 0.18% to 0.27%, the manganese range by weight is 0.70% to 0.95%, the molybdenum range by weight is 0.35% to 0.55%, and the sulfur range by weight is less than 0.001%. The steel alloy has a calcium range, by weight, of 0.0020% to 0.0045%, an aluminum range of 0.010% to 0.080% and a chromium range of 0.2 to 0.3% The alloy may be further include silicon in the range of 0.15% to 0.25%.

[0015] In a further embodiment of the invention, there is provided a casing fabricated from rolled steel for transporting fluids such as oil, gas and steam. The casing has, by weight, a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, and a sulfur range of less than 0.002%. Molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy. This substantially precludes the formation of boron nitride and titanium nitride.

[0016] In another preferred embodiment of the invention there is provided a rolled casing wherein the alloy chemistry is as follows: the carbon range by weight is 0.18% to 0.27%; the manganese range by weight is 0.70% to 0.95%; the molybdenum range by weight is 0.35% to 0.55%; and the sulfur range by weight is less than 0.001%. The steel alloy has a calcium range, by weight, of 0.0020% to 0.0045%, an aluminum range of 0.010% to 0.080% and a chromium range of 0.2 to 0.3%. The alloy may be further include silicon in the range of 0.15% to 0.25%.

[0017] In accordance with a further aspect of the invention, there is provided a method of extracting oil and gas from oil wells. In accordance with this method, a casing fabricated from rolled steel is installed to carry the oil and gas. The casing is made from a steel alloy that has, by weight, a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, and a sulfur range of less than 0.002%. The steel alloy substantially excludes boron and titanium.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A detailed description of the preferred embodiments are provided herein below with reference to the following drawings, in which

[0019]FIG. 1 is a plane view of a hydrogen-induced crack in the IK55 casing product which clearly shows boron nitride and titanium nitride inclusions on the surface of the crack;

[0020]FIG. 2 is a graph plotting the compressive stress applied to a casing made from alloy A of the invention against the resulting percentage compression of the casing; and

[0021]FIG. 3 is a graph plotting the tensile stress applied to a casing made from alloy A of the invention against the resulting percentage elongation of the casing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0022] Conventional steel-making techniques known in the industry including clean scrap melting practices may be used to make an alloy falling within the scope of the invention once the chemistry is known. The product of clean scrap melting frequently contains low concentration of benign alloying elements, trace amounts of miscellaneous elements, some of which may have deleterious effects, and minor alloying elements.

[0023] It was well known prior to the present invention that other benign alloying elements might be added to alloys of the present general type without interfering with the metallurgical objectives of the present invention. Examples of such benign alloying elements are nickel, copper, aluminum and silicon. By controlling the concentration of these alloying elements, one can realize a benefit in the alloy. Accordingly, these may be referred to as minor alloying elements or benign alloying elements depending upon he concentration. For example, copper may be found in concentrations of approximately 0.1% while nickel contents may be as high as 0.1% by weight in scrap.

[0024] It was well known prior to the present invention that trace amounts of miscellaneous elements might be found in typical charges of scrap steel to the melt furnaces, without serious damage to the alloying objectives of the present invention. The present invention as described and claimed does not take into account the possible presence of such trace amounts of miscellaneous elements.

[0025] It was well known prior to the present invention that small amounts of some elements having a potentially deleterious effect on the desired metallurgical objectives of the invention could be present in the scrap charge. Such elements include lead, tin, tungsten, arsenic and phosphorus. The present invention as described and claimed does not take into account the possible presence of such potentially deleterious amounts of harmful elements nor of the countermeasures that might conventionally be taken; it is presumed that if a steelmaker encounters a problem of the foregoing sort, the steelmaker will take effective countermeasures; in the worst case, a particular batch of steel may be scrapped or put to a less demanding end use.

[0026] It is was also well known that a number of minor alloying elements may be present in quantities sufficient to impart an effect on the physical properties of the resultant alloy steel. For example, silicon is well known to be present in the scrap charge. Other alloying elements that can frequently be present in typical charges of scrap steel include aluminum, copper and nickel.

[0027] Accordingly, the description below is directed to the chemistry of the invention without referencing the possible presence of benign alloying elements, miscellaneous elements, deleterious elements, methods to remove deleterious elements or minor alloying elements.

[0028] Through experimentation, the applicant has gained insight into metallurgical factors that contribute to the susceptibility of steel alloys to HIC and SSC. These factors include the presence of large or elongated inclusions in the alloy, as well the presence of titanium nitride and boron nitride precipitates, and the degree of alloy segregation.

[0029] The applicant's analysis of the problem and its solution is as follows:

[0030] 1. Large or Elongated Inclusions

[0031] Large or elongated inclusions act as sinks for atomic hydrogen, which diffuses into the metal from corrosion reactions at the casing surface. At the inclusion-matrix interface, atomic hydrogen combines to form molecular H₂. Gradually, the hydrogen pressure will increase until a crack is initiated and propagates through the metal. Elongated MnS inclusions are particularly susceptible to this form of attack.

[0032] To reduce the MnS inclusion content, the alloy of the present invention employs a clean scrap melting practice to reduce sulfur to 0.001 wt percentage. As well, Ca powder is injected into the molten alloy. Calcium powder preferably combines with sulfur remaining in the steel to form globular CaS particles which float out of the bath. Calcium powder is no longer added when the concentration of calcium reaches a range of 0.0020% to 0.0045%; at that point, sufficient sulfur has been removed to lower the sulfur concentration to an acceptable range.

[0033] 2. Titanium and Boron Nitride Precipitates

[0034] From experimentation, and as illustrated in FIG. 1, the applicant observed that titanium and boron nitride precipitates are frequently present on the crack surfaces of alloy samples after HIC testing, and appear to contribute to the fracture behaviour. Accordingly, the alloy of the present invention excludes these elements; instead the Mo (molybdenum) content is increased to provide hardening.

[0035] 3. Alloy Segregation

[0036] The presence of hard bands associated with alloy segregation appears to contribute to HIC and SSC. The most direct way to minimize segregation is to reduce the percentage of the segregating elements, particularly carbon, manganese and phosphorous. Accordingly, the carbon, manganese and phosphorous contents have been kept relatively low in the alloy of the invention. Through experimentation with various alloys in which titanium and boron have been substantially excluded and molybdenum has been added to provide hardening, it has been found that the increased alloy segregation resulting from increases in the carbon content can be offset by reductions in the manganese content. The chemistry of alloy B specified below has been determined, in part, by this insight.

[0037] Generally, the following ranges of chemistries of the elements mentioned are acceptable in alloys of the invention:

[0038] 1. The acceptable percentage range of carbon is 0.15 wt % to about 0.35 wt % (significant digits=2).

[0039] 2. The acceptable percentage range of manganese is 0.60 wt % to 1.10 wt % (significant digits=2).

[0040] 3. The acceptable minimum percentage of molybdenum is 0.15 wt % (significant digits=2); note that while the alloy can be made using greater than 0.65 wt % molybdenum, to do so would probably be uneconomic; accordingly, the preferred range would not exceed about 0.65 wt %. 4. The acceptable percentage range of sulfur is up to 0.002 wt % (significant digits=3); sulfur should preferably be eliminated to the extent economically possible.

[0041] While all of the alloy chemistries falling within the above-specified ranges will enjoy, to some extent, the advantages of the invention, not all chemistries falling within these ranges are desirable as there are trade-offs that must be borne in mind when selecting an alloy chemistry. For example, while an alloy having a carbon content of 0.35%, a manganese content of 1.10%, and acceptable ranges of sulphur, molybdenum, boron and titanium, would fall within the above-specified range and would enjoy the general advantages of the invention in terms of the reduced susceptibility to HIC due to the exclusion of titanium and boron, the relatively high carbon and manganese content of such an alloy would lead to increased alloy segregation. This increased alloy segregation would reduce HIC and SSC resistance. Accordingly, increases in one of carbon or manganese should be offset by decreases in the other of carbon or manganese. If, on the other hand, the manganese content is kept towards the low end of the range, it will be possible to raise the carbon content to near the upper end of the range, but it will also be necessary to raise the molybdenum content of the alloy to compensate for some of the contribution to hardening that would otherwise be afforded by the manganese. In this case, the need to increase the molybdenum content to substitute for the manganese, can be offset by increasing the chromium content to substitute for the manganese and molybdenum. Thus, while the above-specified ranges are stated in absolute terms, there are complicated interrelationships between the elements that must be borne in mind when selecting a suitable alloy chemistry. However, through reasonable experimentation, those skilled in the art could determine many different suitable alloy chemistries that fall within the above-specified ranges. Two such alloy chemistries are described in detail below.

[0042] Alloy A

[0043] Ipsco Inc.'s alloy A represents a preferred embodiment of the alloy invention and has the following chemistry:

[0044] Essential Alloying Elements:

[0045] 1. The percentage range of carbon in alloy A is 0.15 to 0.22 wt %; the optimum amount is 0.20 wt % (significant digits=2).

[0046] 2. The percentage range of manganese in alloy A is 0.60 wt % to 1.10 wt %; the optimum amount is 0.70 wt % (significant digits=2).

[0047] 3. The percentage range of molybdenum in alloy A is 0.28 wt 0.28% to 0.32 wt %; the optimum amount is 0.30 wt % (significant digits=2).

[0048] 4. Calcium is less than or equal to 0.0045%, and is preferably in the range of 0.0020 to 0.0045 wt % (significant digits=4). (As mentioned above, calcium can be used to combine with trace sulfur to form CaS globules that float out of the bath.)

[0049] Optional Alloying Elements:

[0050] 1. Aluminium is in the range of 0.010 to 0.080% and is preferably in the range of 0.020 to 0.040 wt % and is preferably 0.030 wt % (significant digits=3). Aluminum is added to deoxidize the alloy.

[0051] Undesirable Alloying Elements:

[0052] 1. The percentage of sulfur is <0.001 wt % (significant digits=3).

[0053] 2. Boron and titanium should be present in no more than trace amounts; they should be eliminated to the extent economically feasible.

[0054] An alloy made in accordance with the above chemistry has been found to resist HIC in laboratory environments with a pH as low as 2.68, and in addition, possesses the mechanical properties necessary to resist cracking due to thermal expansion and compression experienced in the horizontal well environment. Specifically, the applicant's alloy A, when rolled into a 7-inch diameter casing, has a thermal expansion coefficient of 14.2×10-6° C.⁻¹ enjoys the properties listed in Tables 1 and 2 below: TABLE 1 Mechanical properties of Alloy A: Elong. Hardness Heat Treatment YS UTS Y/T (%) (VHN) Q & T 602 MPa 662 MPa 0.91 32 245

[0055] TABLE 2 Mechanical properties at elevated temperatures: YS UTS Elong.* Test (MPa) (MPa) Y/T (%) Compression (350° C.) 468 — — Tension (350° C.) 457 642.1 0.71 33.8 Tension (190° C.) after 1% 520 657 0.79 42.8 compression

[0056] Alloy B

[0057] This is a high performance alloy, that is more expensive than alloy A due to the higher content of alloying elements such as molybdenum and chromium, but also offers HIC resistance combined with superior performance in laboratory SSC tests. Generally, the following ranges of acceptable chemistries are suitable for alloy B of the invention:

[0058] Essential Alloying Elements:

[0059] 1. The percentage range of carbon in alloy B is 0.18 to 0.27 wt % (significant digits=2).

[0060] 2. The percentage range of manganese in alloy B is 0.70 wt % to 0.95 wt % (significant digits=2).

[0061] 3. The percentage range of molybdenum in alloy B is 0.35 wt % to 0.55 wt % (significant digits=2).

[0062] 4. Calcium is less than or equal to 0.0045%, and is preferably in the range of 0.0020 to 0.0045 wt % (significant digits=4). (As mentioned above, calcium can be used to combine with trace sulfur to form CaS globules that float out of the bath.)

[0063] Optional Alloying Elements:

[0064] 1. Chromium is in the range of less than or equal to 0.50% and is preferably in the range of 0.10 wt % to 0.50 wt %, and is preferably in the range of 0.20 wt % to 0.30 wt % (significant digits=2). Within the above-defined ranges, the percentage of chromium varies inversely with the percentage of manganese and molybdenum.

[0065] 2. Aluminium is in the range of 0.010 to 0.080% and is preferably in the range of 0.020 to 0.040 wt % and is preferably 0.030 wt % (significant digits=3). Aluminum is added to deoxidize the alloy.

[0066] Undesirable Alloying Elements:

[0067] 1. The percentage of sulfur is <0.001 wt % (significant digits=3).

[0068] 2. Boron and titanium should be present in no more than trace amounts; they should be eliminated to the extent economically feasible.

[0069] In a further preferred embodiment of the invention, the invention is embodied as a well casing made from an alloy as described above. The casing is manufactured using conventional manufacturing technique such as rolling and forming into a 7-inch diameter casing using an electric resistance welding process. This process was well known in the art prior to the time of the invention and was considered to be generally superior to the production of seamless casing as it is more cost effective and produces a pipe with a greater uniformity of wall thickness. It was, however, not generally applied to the production of SSC- and HIC-resistance steel alloys prior to the present invention.

[0070] After welding, the pipe is quenched and tempered. It was well known prior to the present invention that conventional quench-and-temper technology could produce the desired micro-structure of the present invention.

[0071] A well casing in accordance with this embodiment of the invention and made from alloy A as defined above enjoys the mechanical properties listed in Table 3 below. The API 5CT L-80 specification is provided for comparison purposes. TABLE 3 Properties of a production run of Alloy A casing: Elong YS UTS · Hardness Product (MPa) (MPa) Y/T (%) R_(B) HV₅₀₀ 0.395 wall, 9 ⅝″ dia 603 698 0.86 33 96.9 228 API 5CT L-80 552- 655 — 18.5 23 max — Specification 655 min min R_(C)

[0072] The invention may also be implemented as a method of extracting oil and gas from oil wells in which the casing of the above embodiment of the invention is installed to carry oil and gas or in which a well casing is installed made of the alloy of the invention.

[0073] Other variations and modifications of the invention are possible. All such modifications or variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto. 

What is claimed is:
 1. A rolled steel alloy characterized in that the alloy comprises by weight a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, a calcium range of less than or equal to 0.0045%, and a sulfur range of less than 0.002%; wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride.
 2. A rolled steel alloy as defined in claim 1, further characterized in that the carbon range by weight is 0.20% to 0.30%, the manganese range by weight is 0.65% to 0.75%, the molybdenum range by weight is 0.45% to 0.55%, and the sulfur range by weight is less than 0.001%.
 3. A rolled steel alloy as defined in claim 2, further characterized in that the alloy comprises by weight an aluminum range of 0.010% to 0.080%, and a calcium range of 0.0020% to 0.0045%.
 4. A rolled steel alloy as defined in claim 3, further characterized in that the alloy comprises by weight a chromium range of 0.2% to 0.3%.
 5. A rolled steel alloy as defined in claim 4, further characterized in that the alloy comprises by weight a silicon range of 0.15% to 0.25%.
 6. A casing for transporting fluids such as oil, gas and steam, wherein the casing is made from a rolled steel alloy characterized in that said alloy comprises by weight a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, a calcium range of less than or equal to 0.0045%, and a sulfur range of less than 0.002%; wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride.
 7. The casing of claim 6 further characterized in that the carbon range by weight is 0.15% to 0.30%, the manganese range by weight is 0.65% to 0.75% the molybdenum range by weight is 0.45% to 0.55%, and the sulfur range by weight is less than 0.001%.
 8. The casing of claim 7, further characterized in that the alloy comprises by weight an aluminum range of 0.20% to 0.040%, and a calcium range of 0.0020% to 0.0045%.
 9. The casing of claim 8 further characterized in that the alloy comprises by weight a chromium range of 0.20% to 0.30%.
 10. The casing of claim 9, further characterized in that the alloy comprises by weight a silicon range of 0.15% to 0.25%.
 11. The casing of claim 6, further characterized in that the carbon range by weight is 0.18% to 0.27%, the manganese range by weight is 0.70% to 0.95%, the molybdenum range by weight is 0.35% to 0.55%, and the sulfur range by weight is less than 0.001%.
 12. The casing of claim 11, further characterized in that the alloy comprises by weight a chromium range of 0.20% to 0.30%.
 13. The casing of claim 12 further characterized in that the alloy comprises by weight an aluminum range of 0.020% to 0.040%, and a calcium range of 0.0020% to 0.0045%.
 14. The casing of claim 13, further characterized in that the alloy comprises by weight a silicon range of 0.15% to 0.25%.
 15. A method of transporting oil and gas extracted from oil and gas wells wherein a casing is installed to carry the oil and gas, said casing being made from a rolled steel alloy characterized in that said alloy comprises by weight a carbon range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum range of at least 0.15%, a calcium range of less than or equal to 0.0045%, and a sulfur range of less than 0.002%; and, wherein molybdenum is included in the alloy to harden the alloy, so as to enable boron and titanium to be substantially excluded from the alloy, thereby substantially precluding the formation of boron nitride and titanium nitride.
 16. A rolled steel alloy as defined in claim 1, further characterized in that the carbon range by weight is 0.18% to 0.27%, the manganese range by weight is 0.70% to 0.95%, the molybdenum range by weight is 0.35% to 0.55%, and the sulfur range by weight is less than 0.001%.
 17. The rolled steel alloy as defined in claim 16, further characterized in that the alloy comprises by weight an aluminum range of 0.020% to 0.040%, and a calcium range of 0.0020% to 0.0045%.
 18. The rolled steel alloy as defined in claim 17, further characterized in that the alloy comprises by weight a chromium range of 0.20% to 0.30%.
 19. The rolled steel alloy as defined in claim 18, further characterized in that the alloy comprises by weight a silicon range of 0.15% to 0.25%. 